In the placebo group, a normal saline bolus dose 0.3ml/kg will be iv infused followed by a continuous infusion of 0.1 ml/kg/h. In the MgSO4 group, a MgSO4 bolus dose 0.3mL/kg will be iv infused followed by a continuous infusion of 0.1 ml/kg/h. infusion terminated by the end of surgery. Post operative emergence agitation will be assessed by using the Pediatric Anesthesia Emergence Delirium scale.